"Students were never in any danger," said D.C. Public Schools in a statement. "Staff at both schools quickly followed emergency protocols in response to the incidents."

Preliminary testing of the letters by the D.C. Fire Department determined that they didn't contain harmful substances. The letters have been sent to a joint FBI/CDC regional lab, and will then go to an FBI lab at Quantico for additional testing, as is protocol.

Authorities have not yet determined if the two letters are connected to similar cases at schools around the country.

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